1. Field of the Invention
This invention generally relates to a thermal printhead suitable for use in facsimile machines and printers, such as video printers and handy printers, and, in particular, to an edge type thermal printhead having a plurality of heat-producing (resistor) elements formed in the form of a single array at one end surface thereof
2. Description of the Prior Art
FIG. 1 schematically illustrates the main portion of a conventional edge type thermal printhead. As shown, the edge type thermal printhead includes a substrate 1 typically comprised of alumina ceramics and serving also as a heat sink. A glass glaze layer 2 is formed at the bottom end surface of the substrate 1 as a heat insulator, and an array of heat-producing elements or resistor elements 3 is formed on the glass glaze layer 2. Electrodes 4 and 5 for passing current to the array of heat-producing elements 3 selectively are also formed on the substrate 1. Typically, a protective layer is formed on the array of heat-producing elements 3, but such a protective layer is omitted in FIG. 1. Such an edge type thermal printhead has the following advantages.
(1) An excellent contact between a recording medium and the thermal printhead may be achieved, and the surrounding input device may be simplified.
(2) Since the array of heat-producing elements 3 are provided in the form of projections at the end surface of the substrate 1, a uniform contact pressure with a recording medium and/or an ink ribbon may be easily attained, and, in particular, a printed image of high quality may be attained in a transfer type thermal recording system in which use is made of a heat-sensitive ink ribbon.
(3) Printing may be easily effected on a plate-shaped recording medium.
(4) A multi-thermal printhead for use in color printing may be easily implemented.
(5) The diameter of a platen roller may be set independently of the outer shape of the substrate 1.
However, in such an edge type thermal printhead, since the array of heat-producing elements 3 is formed at an end surface of the substrate 1, it has various disadvantages as will be enumerated below. (i) Since heat-producing elements are formed on an end surface of a substrate, only two rows of such heat-producing elements may be formed. For this reason, surface-to-surface contact mounting cannot be effected and it is not suited for mass production. (ii) An end surface of a substrate must be subjected to glass glaze processing, which is technically difficult and high at cost. (iii) A pattern of the heat-producing elements 3 and the electrodes 4 and 5 must be formed extending between an end surface and a side surface over an edge. Difficulty will be encountered in resist application, photomask formation and exposure, and the number of steps in the formation of such a pattern will be increased.